They assume that all orders are delivered in a preplanned customer sequence and constraints in their model include a
constant lifespan for the perishable product, customer requirements regarding quantity and time windows within
which the delivery must be received. In this model, not all demands can be satisfied; hence, the objective is to find
the subset of the customers which can be served to maximize the demand that can be met. The authors show that this
problem is NP-hard and a branch-and-bound search algorithm as well as a heuristic lower bound is developed